1. Field of the Invention
The present invention relates to a power supply circuit for a digital camera, and more specifically, to a power supply circuit for the digital camera so configured as to cause no voltage drop even at the time of activation of LEDs.
2. Description of the Related Art
As auxiliary light for an image inputting device of a digital camera, a strobe utilizing an arc tube in which emitted light quantity is large for example has been widely used in a camera. On the other hand, LEDs (Light Emitting Diodes) having large luminance have recently come to be available by development of semiconductors. The LED has an advantage over the strobe light in that color temperature is adjustable in a case of the LED if three colors of R, G and B are used, and thereby irradiation of almost perfect white is possible.
However, a plurality of LEDs has to be used for the emission in order to obtain the light quantity equivalent to that of the strobe conventionally used, and therefore, a large amount of current flows when the LEDs operate. In such case, a voltage drop occurs particularly if a battery having large internal resistance, such as an alkaline cell is used. This may cause the power supply supplied to the entire digital camera to generate power supply voltage lower than that sufficient for the camera to operate, so that the image inputting device becomes unable to operate thereby.
Particularly, the luminance, the color, etc. are unstable in the early stage of lighting of the LED. Accordingly, it is necessary to flow more current at the time of activation of the LED than in a steady state so as to reduce start-up time, hence the voltage drop becomes severer. Therefore, measures against such problem have to be taken.
For example, JP-A 2002-116481 discloses that LEDs 2101 of R, G and B (2101R, 2101G and 2101B) are used as a strobe light source, as shown in FIG. 5. According to the disclosure, electrical energy stored in a capacitor 2102 is supplied to each of the LEDs 2101, and a system controller 2103 controls a quantity of light emission of each of the LEDs 2101 so as to reach color temperature set manually by a color temperature setting volume 2104 or color temperature of a photographing field detected by a color temperature sensor 2105.
It teaches that, accordingly, the color temperature in a color of light emission can be changed manually or automatically by using the strobe light source which is capable of adjusting the color temperature, such as the LEDs of R, G and B.
Also, JP-A 2003-215674 discloses that the LEDs 2101 of R, G and B (2101R, 2101G and 2101B) are used as a strobe light source, likewise as shown in FIG. 5. According to the disclosure, a number ratio among three-color light emitting elements is set corresponding to a ratio of emitted light quantities of the respective light emitting elements of the LEDs 2101 so that the color temperature of strobe light by the three-color light emitting elements may become a desired one.
It teaches that a new stroboscopic device for camera using the LED or the like can be provided, which is capable of easily control the emitted light quantity of the respective light emitting elements of R, G and B.
Meanwhile, a more advantageous effect is obtained if the LED and the strobe are combined together and either one of them or both is used according to a condition of photographing. It is possible to carry out the most appropriate photographing according to the condition of photographing, for example, by arranging and lighting the LED adjacent to a photographing device when performing close-photographing, or by using the strobe when performing distant-view photographing. In such case, if power supplies are provided separately for the LED and for the strobe, a size of the device becomes large and thus convenience of user is impaired.
According to the proposals made in the above-mentioned JP-A 2002-116481 and JP-A 2003-215674, however, since a battery 2106 and a boosting up-converter (voltage up-converter) 2107 are directly connected to each other for the power supply as shown in FIG. 5, they cannot solve the above-mentioned problem that it is necessary to flow more current at the time of activation of the LED than in the steady state, which causes severer voltage drop.
In addition, with regard to the point mentioned above that more advantage effect can be obtained if the LED and the strobe are combined and either one of them or both is used according to the condition of photographing, both JP-A 2002-116481 and JP-A 2003-215674 use only the LED and do not disclose the combined use of the LED with the strobe.